The invention relates to a telephone set for an integrated services digital network (ISDN) comprising a main circuit at least for transmitting signals between a handset with a cord and a line terminating unit which can be coupled to an interface for the integrated services digital network.
The integrated services digital network (ISDN) makes a plurality of communication services between integrated standardized user interfaces possible. For transmitting narrow-band messages (speech, data and text communication etc.) over a line, two B-channels having each a capacity of 64 kbit/s are available. A D-channel with 16 kbit/s is provided as a signalling channel for transmitting switching information signals. In the subscriber area of an integrated services digital network an So-interface is available to which a telephone set can be connected.
The telephone set mentioned above for an integrated services digital network is known from the publication entitled "ISDN Komfortsprechapparat SET 50 mit integrierter Datenschnittstelle" by P. Hirth and B. Lurz, PKI Technische Mitteilungen, Vol. 1, 1988, pp. 19 to 22. The telephone set is coupled to an So-interface of the integrated services digital network by means of its line terminating unit. The line terminating unit and also a circuit for controlling keypad and an indicator and the speech processing circuit are controlled by a main control circuit over a bus system, over which speech signals, control signals, address signals etc. are transported. The speech processing circuit in which a handset with a cord and a loudspeaker/microphone combination for handsfree operation are available, comprises an analog-to-digital converter and a digital-to-analog converter. Furthermore, the circuit comprises an adapter circuit connected to the bus system and which is capable of transmitting or receiving data by means of a data interface (for example, V24 interface). With such a telephone set the advantages of the integrated services digital network can be made use of when a telephone call is made. However, the handset with a cord is connected to the stationary section of the telephone set, so that the telephone subscriber is restricted in his movements.
A telephone set with a mobile handset (cordless telephone set) which may be known from the article entitled "PATSY--ein schnurloses Komforttelefon" by J. Hoflinger and G. Ranner, PKI-Technische Mitteilungen, Vol. 2, 1988, pp. 7 to 14, increases the freedom of movement when having a telephone conversation. Such a telephone set with a mobile handset does not have any So interface for the integrated services digital network, but an analog a/b interface. An adapter for adapting the a/b interface to the So interface could be inserted. Such an adapter, however, is not inexpensive. Furthermore, when the mobile handset is used it is not possible to use the advantages of the integrated services digital network when used with a telephone set comprising a handset with a cord.